I believe the post/thread Lanzfeld is referring to is this one:
http://www.subsim.com/radioroom/show...6&postcount=12
Ducimus says the visual range is 30km, but points out that that is for flawless conditions. Environmental variables (light, sea state, fog, clouds etc.) all have a dramatic impact on this.
See also, Kpt. Lehmann's comments on the same subject:
http://www.subsim.com/radioroom/show...10&postcount=7